A typical color cathode-ray tube (CRT) includes a funnel with an attached faceplate. A cathodoluminescent display screen is carried on the inner surface of the faceplate. The display screen comprises a pattern of discrete phosphor deposits that are arranged in sets of three colors. Three electron guns are mounted within the CRT funnel and aimed toward the display screen. The guns are controlled by an associated scanning system to direct electron beams toward the phosphor deposits. The electron beams strike the phosphor deposits after passing through a shadow mask that is positioned between the electron guns and the phosphor deposits. Each electron gun produces a single electron beam for exciting only one color in each set of phosphor deposits as the beam is swept across the display screen by the scanning system.
A shadow mask is a multi-apertured membrane of electron beam absorbing material. The apertures in the shadow mask have the same shape as the phosphor deposits. The shadow mask is precisely positioned adjacent to the display screen of the CRT, and the apertures are arranged so that an electron beam can strike only one color in each set of phosphor deposits as the beam is swept across the screen. Accordingly, the shadow mask shadows or screens the remaining phosphor deposits from that particular beam.
The shadow mask may be constructed so that it is under tension, or "tensed". A tensed shadow mask resists expansion out of its original plane when the mask is heated by the electron beams.
The pattern of phosphor deposits on the display screen is typically formed by a direct photoprinting process. This process includes coating the display screen with a photosensitive slurry containing phosphor particles of one color, for example, red. Next, the shadow mask is removably mounted adjacent to the display screen, and the coating is exposed to ultraviolet light that is projected through the mask apertures. The light emanates from a source that is located at a position that corresponds to the position later assumed by the electron gun that is used to excite the red phosphor deposits. After the display screen is exposed to ultraviolet light, the shadow mask is removed and the display screen coating is treated to remove the unexposed portions of the photosensitive slurry so that a pattern of red phosphor deposits remains. The above-described photoprinting process steps are then repeated for the remaining two colors of phosphor deposits.
It can be appreciated that during the photoprinting process, the shadow mask serves as a stencil for defining the location of phosphor deposits. Accordingly, the shadow mask must be precisely positioned in the same location relative to the display screen each time the shadow mask is mounted to serve as a stencil for the production of one color of the phosphor deposits. One method of ensuring that the tensed shadow mask is precisely positioned involves anchoring the shadow mask to a frame, and providing the frame with a registration mechanism that cooperates with another registration mechanism on the CRT faceplate. For example, U.S. Pat. No. 4,595,857 discloses a registration mechanism that includes three cavities formed in a peripheral sealing land of the CRT faceplate. Correspondingly located and shaped cavities are formed in a rigid frame to which a tensed shadow mask is affixed. The frame overlies the inner portion of the sealing land with the cavities in the frame facing the cavities in the faceplate. A rigid sphere is seated within each void formed by the three sets of facing cavities. The spheres rest against the cavity walls and secure the shadow mask frame (hence, the shadow mask) in a particular position relative to the display screen during the photoprinting process described above.
The CRT funnel, shadow mask frame, and faceplate are permanently sealed together after the photoprinting process is complete. To seal the funnel, frame, and faceplate, the voids formed by the facing cavities are filled with frit so that the spheres are nested within the frit. The frit is also applied between the outer portion of the sealing land of the faceplate and the CRT funnel surface to which the faceplate is sealed. After the frit is applied, the assembled CRT funnel, faceplate, and shadow mask frame are heated to approximately 430.degree. C. for approximately 45 minutes. During heating, the frit devitrifies. Upon subsequent cooling, the frit seals the CRT funnel, faceplate, and shadow mask frame together.
It has been found that with prior techniques, the registration between the shadow mask and the display screen, which registration may be precisely maintained during processing of the display screen, changes after the CRT is finally sealed. Such changes in the registration between the shadow mask and the display screen may result from an uneven application of frit material, or from small amounts of frit remaining trapped between the spheres and cavity walls that comprise the registration mechanism described above. Any registration change after the photoprinting process will have deleterious effects on the color purity of the CRT because the electron beams will not precisely strike their associated color of phosphor deposits.
It is desirable to provide a shadow mask assembly that is adapted for repeated precise registration relative to a display screen in order to facilitate processing of the phosphor deposits on the display screen, wherein the registration of the shadow mask relative to the display screen is unaffected by the subsequent permanent sealing of the CRT.